1. Field of the Invention
This invention relates to a method of locating thrombi in humans and animals, and more particularly to the use of a radiolabeled peptide derived from the degradation of crosslinked fibrin by plasmin as an imaging agent for locating thrombi in vivo.
2. Description of the Prior Art
Disorders of the blood clotting system are present in a significant fraction of the human population. The most common such disorder is the formation of thrombi, clots formed in a blood vessel or heart cavity that remain at the point of formation. Thrombi in heart vessels, for example, can restrict blood flow, resulting in myocardial infarction (death of heart muscle), one of the most severe forms of heart attacks.
In addition, parts of a thrombus of the entire thrombus can dislodge from its point of attachment and move through the blood vessels until it reaches a point where the passage is restricted. The resulting sudden blockage of blood flow is known as a thromboembolism. One part of the circulation system particularly subject to emboli formation is in the lungs, the first point at which main arteries divide into smaller arteries and capillaries after the heart has received blood from the venous system. A 1968 study of all hospital deaths showed that pulmonary emboli were present in 50% of patients who died at age 60, and in 64% of those who died at age 70. In the patients with emboli, an embolus was the major cause of death in 43% of the cases. Overall, over 700,000 cases of pulmonary emboli are detected in the United States every year and more than 90% of these emboli can be traced to deep vein thrombosis.
Accordingly, methods which enable thrombi to be detected are of great medical importance so that preventive measures, such as anticoagulant therapy or surgery, can be taken.
In recent years, human fibrinogen labeled with a radioisotope has been used for the detection of thrombi in the deep veins of the leg and in other parts of the body. Fibrinogen can be labeled with iodine-125 (U.S. Pat. No. 3,9833,996) or technetium-99 m (U.S. Pat. No. 4,057,617) and injected via a suitable carrier into a vein where it enters into clot (thrombus) formation. Activation of fibrinogen by the enzyme thrombin causes the release of fibrinopeptides (fibrin monomers), which polymerize to form a fibrin polymer that forms part of a clot or thrombus. When radiolabeled fibrinogen enters into clot formation, the radioactivity becomes localized and the thrombus can be located by external detection of the radiation.
Although the use of radiolabeled fibrinogen has constituted an advance in the location of thrombi, some problems still exist. Fibrinogen is taken up only by relatively fresh or still forming thrombi and hence may not be sufficiently localized in old thrombi (&gt;1 day old) to allow effective external imaging. Accordingly, an imaging agent that would be taken up by both forming thrombi and previously formed thrombi is highly desirable. However, no such agent was known prior to the present invention.